1. Field of the Invention
The present invention relates to monofilaments made of poly(ethylene terephthalate) (PET) with good and/or improved loop mechanical properties as well as good and/or improved tensile mechanical properties, as well as process for producing such monofilaments. The monofilaments are preferably configured and/or utilized for industrial fabric applications and can take the form on load bearing yarns that resist loop failure and resist tensile creep at high temperature.
2. Discussion of Background Information
Poly(ethylene terephthalate) (PET) filaments of high strength are well known in the art, and are commonly utilized in industrial applications including being utilized as reinforcement members in conveyor belts, tire cords, reinforced rubber, and paper machine clothing.
Significant efforts have been made to establish the mechanical properties required for industrial applications. Among other properties, high tensile strength and high modulus are most demanding. Monofilaments are typically required to bear high load, and to resist deformation (creep) in various applications. Those skilled in the art fully realize that the high strength and modulus for the filaments can be achieved by stretching the extrudates in order to orient the polymer microstructure. A subsequent annealing processes can further improve strength and modulus by repairing defects generated during the stretching process, and by inducing higher crystallinity of the final filaments.
The high stretching and high temperature annealing process, however, can cause a brittle failure mode in the filaments when subjected to high loads, i.e., greater than about 11 pounds/inch (lbs/in). The brittle failure can be manifested more readily in a loop breaking mode. Under tension, a looped filament can easily become fibrillated leading to catastrophic break at the loop area. FIG. 1 shows a typical example of such a failure at a loop area of a PET monofilament.
Toughness or total work/energy required to break a filament (rather than strength) is a better measure for ultimate filament mechanical durability. It combines both strength and elongation of the tested sample, and is conveniently calculated as the area underneath the stress-strain curve at testing. Filaments having a brittle failure mode would typically have a very low toughness, especially loop toughness.
Many efforts, industrial and academic alike, have been made in achieving high toughness PET filaments. For example, U.S. Pat. No. 4,867,936 (1989) describes a process for producing such filaments primarily by reducing polymer degradation during melt processing. The maximum tensile toughness achieved is up to 0.67 grams-force/denier (gf/den). EP 1,887,111 discloses additional efforts that have been made in increasing the drawing of PET by the use of additives.
Although not directed to PET, U.S. Pat. No. 5,405,695, the disclosure of which is hereby expressly incorporated by reference in its entirety, describes a process for achieving high toughness in filaments of a different semi-crystalline polymer. The process, however, is believed by the inventors of the instant application to be applicable to the production of PET filaments. This patent especially emphasizes the skin-core structure effect of filament on the toughness. Large diameter monofilaments at a diameter range of 0.1-1.5 mm typically have a much poorer toughness compared to multi filaments at a diameter range of a few microns. Fine diameter fibers have a much smaller difference between skin bending and the center (natural line) bending than large diameter monofilaments. In small diameter fibers, microstructure defects are also minimized—further contributing to their superior properties.
Continuous improvements have been required in high strength, high toughness industrial monofilaments to make them suitable for use as load bearing yarns for industrial fabrics in demanding applications.